Department of Earth and Environmental Sciences Division of Soil and Water Management The implementation of bioavailability in defining PNEC values for trace metals and metalloids in soil Erik Smolders
Zn dose (mg/kg) 0 30 150 300 450 750
Main message in this talk 1. Metal toxicity depends on chemical properties of the soil. Data and models are needed to express the PNEC as a function of soil properties, i.e. regression slopes correct for differences among soils 2. Metal toxicity in laboratory spiked soils is generally larger than in the field because of lack of full equilibration in the lab (no ageing) and because metal salts induce artefacts that only disappear after sufficient leaching of the soil. A leaching-ageing correction is needed to make toxicity field relevant
Defining the thresholds 1.Standard laboratory toxicity tests with metal salt spiked soils Low High 2. Field experiments 3. Gradient studies in contaminated High Low fields relevance reproducibility clarity of causality
Most ecotoxicological soil limits (plants, invertebrates and microorganisms) based on standard ecotoxicological tests are within the natural background range HC 5 EU RAR* range of geogenic background Zn 26 mg/kg (added) 5-150 mg/kg Cu ~30 mg/kg 2-50 mg/kg Ni 10 mg/kg 1-100 mg/kg Pb 84 mg/kg 5-100 mg/kg *EU Risk Assessment Report; limits without correction for biovailability NOEC Background 0.1 1.0 10 100 1000 Soil Ni (mg/kg)
Why total metal concentrations in soil are no robust index for available dose in toxicity tests 1.Speciation and mobility is different in different soils. Example: a soil with almost 100% sand and very little organic matter cannot immobilise metals, toxicity is very high; the opposite is true in a soil with larger % clay and organic matter 2.Short term effects in spiked soils overestimate long-term effects
Spiked soils are no good model for field contaminated soils because of differences in metal speciation Example: ageing reactions A. Diffusion in micro pores B. Inclusion in crystal structure of soil minerals C. Occlusion by precipitates D. Precipitation on soil surface E. Occlusion in organic matter
Starting idea: test toxicity differences between (1) (2) (3) different spiked soils aged&spiked field contaminated soils and spiked soils 150 150 150 field Response % 100 50 Response 100 50 freshly spiked spiked and 15 months aged Response 100 50 spiked soil 1 soil 2 0 10 100 1000 10000 Total soil metal (mg Zn/kg) 0 10 100 1000 10000 Total soil metal (mg Ni/kg) 0 10 100 1000 Total soil metal (mg Cu/kg) Zn toxicity to plants Ni toxicity to plants Cu toxicity to microbial In 2 different soils with or without ageing activity in spiked or 70 year old contaminated soil Smolders et al. Environ Toxicol Chem, 2009
1. Toxicity of metal between different freshly spiked soils
150 Response % 100 50 soil 1 soil 2 0 10 100 1000 10000 Total soil metal (mg Zn/kg)
NOEC (added Zn, mg Zn/kg) of spiked soils Soil plant growth nitrification glucose respiration maize residue respiration 1 60-240 120 3 120-30 >720 5 3200 400 >1200 200 6 257 257 800 469 8 600 50 100 >1200 9 200 50 400 50 11 425 424 1300 1300 12 700 38 600 >1800 13 199 206 1400 1400 14 600 75 300 >1800 15 600 160 50 38 17 1200 300 100 150 18 1200 150 100 600 19 1000 300 100 150 22 150 75 100 300 Larger effect of soil type on NOEC than that of species
Tomato shoot growth in Cu salt spiked soils y=0.96x+1.47 R 2 =0.75 ecec= cation exchange capacity at soil ph; increases with % clay, %OM and ph
Folsomia candida reproduction in Ni spiked soils log ecec (cmol c /kg)
Interpretation of effects of soil properties for toxicity of cationic metals, e.g. Cu 2+ soil solids soil solution biota M 3 -OH -OH -OM -OM -OH -OH -OM -OH -OH H + H + 1 2 4 M 2+ M-DOM HO- HO- MO- HO- MO- HO- fixed labile metal metal Total metal in soil The terrestrial Biotic Ligand Model (t-blm): ion competition effects (nr. 2) are an extension of the free ion activity model (FIAM) t-blm has been used to model data of freshly spiked soils Thakali et al. 2006 Environ. Sci. Technol.
Effects of soil properties on toxicity can be modelled with t-blm, however this model is too complicated to be adopted, therefore toxicity is normalised with the relative effect of soil property on toxicity, the slopes
Summary of all slopes for different metals and different species Slope=0.96
Using slopes to make limits depending on properties Example: limit is made for a reference soil (average properties in EU), e.g. soil with ecec=10 cmolc/kg Soil ecec ecec NOECmeasured NOEC in reference soil tested reference mg/kg mg/kg soil soil 1 5 10 90 180 2 10 10 200 200 3 20 10 420 210
Experimental ageing of spiked soils 2&3. Toxicity of metals as a function of ageing after spiking and difference with field contaminated soils METHODS Cu contaminated site at Hygum (Denmark)
Toxicity of Ni to barley root elongation 150 Response 100 50 freshly spiked spiked and 15 months aged 0 10 100 1000 10000 Total soil metal (mg Ni/kg) Rooney et al. unpublished
Toxicity of Cu in freshly spiked soils exceeds that in field contaminated soils Field transect Fresh spike
150 Response 100 50 freshly spiked leaching-ageing factor aged 0 10 100 1000 10000 Total soil metal (mg Ni/kg)
Summary of 110 leaching-ageing factors = EC10 EC10 aged, add freshlyspiked, add Horizontal lines: the empirical ageing factor selected for risk assessment. Example for Cu: factor 2
Soil chemical reactions explain the leaching and ageing effects Example: experimental ageing decreases the fraction of added Zn that remains labile as detected by isotope exchange Buekers et al. 2007, Eur. J. Soil Sci.
Larger mobility of metals in metal salt s-spiked soils compared to field contaminated soils can be related to difference in soil ph 9.0 8.0 ph in soil solution 7.0 6.0 5.0 4.0 3.0 spiked soils spiked & leached soils field transect soils 1 10 100 1000 10000 100000 Total Pb in soil (mg/kg) Source of Pb in the field is often PbO, in the lab it is PbCl 2
soil solids soil solution biota M 3 -OH -OH -OM -OM -OH -OH -OM -OH -OH H + H + 1 2 4 M 2+ M-DOM HO- HO- MO- HO- MO- HO- fixed labile metal metal Total metal in soil
Implementation of bioavailability in soil limits
Leaching-ageing (L/A) factors Cadmium 1.0 Copper 2.0 Lead 4.2 Zinc 3.0 Nickel 1 3 (increasing with ph) Cobalt 1 3.5 (increasing with ph) Example for Cu Species Soil NOECadded NOECadded, aged mg Cu/kg mg Cu/kg 1 A 100 200 2 B 200 400 3 B 250 500
C b =background Cu in soil Example for Cu Spec. Soil NOEC added NOEC added,aged C b NOEC total,aged mg Cu/kg mg Cu/kg 1 A 100 200 10 210 2 B 200 400 20 420 3 B 250 500 20 520
NOEC reference = NOEC test abioticfactor abioticfactor reference test slope Reference: scenario for which threshold values must be derived Test: abiotic factors of the soil in which the NOEC or EC10 was derived Slope: slope of regression equation between log ECx and log soil properties
Species sensitivity distribution 100 Frequency of NOEC/EC10 values (%) 80 60 40 20 not normalised normalised to ecec=5 normalised to ecec=35 PNEC 5 0 1 10 100 1000 10000 Soil total Ni concentration (mg/kg) PNEC (Predicted No Effect Concentration) = 5 th percentile/ assessment factor
The biovailability corrections in the EU risk assessments Metals Data normalized with Leaching-Aging factor (L/A factor) Zn 2+ ecec, background Zn, 3 ph Cu 2+ ecec, %clay, %OC, 2 ph Ni 2+ ecec 1-3 (increasing as a function of ph) Cd 2+ - 1
Zn Predicted No Effect Concentration PNEC (mg total metal/kg dry soil) Median sensitive soil * Highly sensitive soil* 24 (added) 28 (total) 94 (added) 111 (total) Weakly sensitive soil * 246 (added) 286 (total) Cu 30 93 162 Ni 8 36 93 Cd 1.1-2.2 Soil properties:cec: 4, 15, and 35 cmol c /kg; ph: 4.5, 5.5, 7.0; %OC 1.0, 2.9, and 12%, %clay 5, 15, and 30% background Zn 8, 51, and 155 mg Zn/kg.
PNEC calculator tool Based on all data and procedures of the EU RAR for Cd, Cu, Pb, Ni and Zn (so far) Calculates soil limits as a function of soil properties (ph, %OM, % clay, ) Microsoft Excel format Available at koen.oorts@arche-consulting.be
Main message in this talk 1. Metal toxicity depends on chemical properties of the soil. Data and models are needed to express the PNEC as a function of soil properties, i.e. regression slopes correct for differences among soils 2. Metal toxicity in laboratory spiked soils is generally larger than in the field because of lack of full equilibration in the lab (no ageing) and because metal salts induce artefacts that only disappear after sufficient leaching of the soil. A leaching-ageing correction is needed to make toxicity field-relevant